Getting From Point A to Point B

Generally, the ability to configure and implement motion-control profiles electronically provides a more powerful and flexible solution than previous mechanical solutions, says Corey Morton, product manager with B&R Industrial Automation Corp. (www.br-automation.com) , Roswell, Ga.

“Always, position is what you’re trying to optimize. Through position, you have to define velocity and acceleration at various points along the move,” explains Carroll Wontrop, senior system engineer with Danaher Motion (www.danaher.com) , another automation components vendor in Radford, Va. “Typically, what a user wants to know is how to get from point A to point B in a certain amount of time.”

In a typical transfer application in the automotive industry, for example, product moves from one position to another in a single direction, either horizontally or vertically, depending on the specific application, B&R’s Morton says. “These types of movements are commonly referred to as point-to-point movements involving only a single axis of motion.”

Know your profile

In these motions, types of profiles include trapezoidal, S-curve, sine-curve, macro and cam-based. The simplest is trapezoidal, says Danaher’s Wontrop. It’s comprised of linear acceleration up to a certain speed or velocity, movement at that constant speed for some distance and then linear deceleration. Through programming language or a table in which each row is a different profile or part of the profile, users must define these three elements for the motion controller.

Called jerk control by some, the curved profiles smooth the transition from acceleration/deceleration to constant speed, Morton explains. To define this non-linear transition, the end-user also must define the shape of the curves and then input that into the controller.

The cam-based profile reveals more complex distance-speed relationships. “You have to define it (the relationship) by a table that has a lot of points on it,” Wontrop states. Typically,cams find use when one motion must be linked—and, thus, synchronized—with another motion. But, he advises, when end-users size the motor drive and controller for this type of motion, ensure that that equipment has sufficient power.

One example of a cam-based move is a conveyor belt with items moving down the belt, but the process requires the belt to stop and a robotic arm to then place something else on that belt, Wontrop says. “If you want to get fine-tuned with acceleration profile, then you can do this with a cam table. It contains input points based on time or input position; and output points based on position.”

But for this, the end-user must determine how many distance outputs match distance inputs. “It may be that for the first three inches of input, output may be two inches, and so on. You can tailor this any way you want. But you’ve got to make sure the motor drive can make the load,” Wontrop explains.

With simple point-to-point moves, an option lies between the curving and cam profiles, which provides the ability to cascade one profile on top of another: the macro profile. It begins and ends, generally, with zero acceleration and deceleration, Wontrop states. “You start out at one speed, go for some distance, then accelerate up to a higher speed, then even decelerate or go to zero and then perhaps have another acceleration.”

In terms of ease of creation of profile, Wontrop notes, in descending order, it’s trapezoidal, S-curve or sine-curve, macro and then cam-based. “For a given acceleration/deceleration, maximum velocity and target position, the motion controller will generate a velocity profile to execute the movement as quickly as possible within given parameters,” Morton adds. But, Wontrop reminds, generating profiles requires end-users to input data that represent the speed/position points from beginning to end of each respective profile. 

C. Kenna Amos, ckamosjr@earthlink.net, is an
Automation World Contributing Editor
More in Control